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Inventory - Additional Information (Detail) $ in Millions	Sep. 30, 2018 USD ($)
Inventory Disclosure [Abstract]
Zero-cost inventory	$ 21.3

X

- Definition

Zero cost inventory.

+ References

No definition available.

+ Details

Name:	alny_ZeroCostInventory
Namespace Prefix:	alny_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- References

No definition available.

+ Details

Name:	us-gaap_InventoryDisclosureAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration